The Applicant has developed a range of Memjet® inkjet printers as described in, for example, WO2011/143700, WO2011/143699 and WO2009/089567, the contents of which are herein incorporated by reference. Memjet® printers employ a stationary pagewidth printhead in combination with a feed mechanism which feeds print media past the printhead in a single pass. Memjet® printers therefore provide much higher printing speeds than conventional scanning inkjet printers.
An inkjet printhead is comprised of a plurality (typically thousands) of individual inkjet nozzle devices, each supplied with ink. Each inkjet nozzle device typically comprises a firing chamber having a nozzle aperture and an actuator for ejecting ink through the nozzle aperture. The design space for inkjet nozzle devices is vast and a plethora of different nozzle devices have been described in the patent literature, including different types of actuators and different device configurations.
Satellite droplets (or ‘satellites’) are a perennial problem in the field of inkjet printheads. When an ink droplet is ejected from a nozzle aperture, there is a tendency for the tail of the droplet to break up into one or more trailing satellite droplets, each having a volume smaller than the main droplet. If ink droplets are ejected perfectly perpendicularly with respect to a nozzle plate of the printhead, then the satellites will tend to land on print media at the same position as the main droplet, causing minimal print artefacts.
However, inkjet nozzle devices usually have an inherent degree of asymmetry, which means that ink droplets may be ejected somewhat skewed from the nozzle plate of the printhead. With skewed droplet ejection, satellite droplets tend to land on print media at a different position than the main droplet and this causes a reduction in print quality.
Hitherto, most attempts to minimize the effects of satellites have focused on compensating for asymmetry in the nozzle device. For example, U.S. Pat. No. 7,780,271, the contents of which are herein incorporated by reference, describes an inkjet nozzle device having a heating element which is offset from the nozzle aperture. The offset compensates for asymmetric bubble formation in the firing chamber and enables non-skewed droplet ejection.
U.S. Pat. No. 8,267,501 (assigned to Eastman Kodak Company) describes inkjet nozzle devices having multi-lobed nozzle apertures. These multi-lobed nozzle apertures are reported to provide advantages, such as straighter trajectory, shorter tails, better accuracy, smaller ink volume, and less satellite effects.
For the sake of completeness, U.S. Pat. No. 5,812,159 (assigned to Eastman Kodak Company) describes inkjet nozzle devices comprising a heating bar extending across the nozzle aperture. The heating bar actuates droplet ejection by lowering the surface tension of an ink meniscus. With a positive ink pressure, this reduction in surface tension at the meniscus enables controlled droplet ejection upon actuation of the heating bar.
It would be desirable to provide an inkjet nozzle device, which minimizes satellite droplet formation and improves print quality.